Reducing the cover-to-diameter ratio for shallow tunnels in soft soils

Abstract

Despite the fact that shallow tunnels have the benefits of low short-term construction costs and long-term operational costs primarily due to the shallow depth of the station boxes, the limited understanding of shallow tunnelling in soft soils is an obstacle to the development of shallow tunnels in urban areas. This study carries out a theoretical investigation of the effects of reducing the cover-to-diameter ratio C/D for shallow tunnels in soft soils. In stability analysis, the uplift, face stability and blow-out mechanisms are investigated. This study investigates interactions between the TBM and surrounding soil in tunnelling process, the stability of the TBM is not taken into account. The relationship between the C/D ratio and the required thickness-to-diameter ratio d/D as well as the required support pressures will be derived in various soils. Ranges of support pressures are also estimated for the TBM. Structural analysis is carried out for the variation of deformations and internal forces of the tunnel lining when reducing the C/D ratio. Since the conventional design models are not suitable in the case of shallow tunnels a new structural analysis model, which includes the difference between loads at the top and at the bottom of the tunnel, is proposed. Optimal C/D ratios with various d/D ratios for shallow tunnels in soft soils are also derived. With respect to ground movement analysis, this research investigates the areas affected by shallow tunnelling with a preliminary assessment of the risk of building damage by investigating surface and subsurface soil displacements. These areas are determined for different tunnel diameters in various soil types and are then compared to recent studies. The total volume loss is estimated at the tunnelling face, along the TBM, at the tail and includes long-term consolidation settlements. By combining empirical models from the literature and the proposed new models, the volume loss components are estimated both for short-term construction and for the long-term consolidation effects. This shows that a no volume loss is feasible in shallow tunnelling with careful control of the support pressure. The boundaries of the influence zones in shallow tunnelling are identified and discussed on the basis of various case studies. The effects of the soil parameters on the influence areas are also investigated. From these calculations, the limits and optimal C/D ratios for shallow tunnelling are deduced and recommendations and solutions for improving the shallow tunnelling process are proposed in this dissertation.